MRI Image Weighting and Contrast PDF

Image weighting and contrast Hayder Jasim Taher PhD of Medical Imaging Outline of my presentation ✓ T1 recovery. ✓ T2 decay. ✓ Relaxation in different tissues. ✓ Image contrast. ✓ Contrast mechanisms. ✓ T1 contrast. ✓ T2 contrast  T1 Recovery ( spine-lattice energy transfer) The...

Image weighting and contrast Hayder Jasim Taher PhD of Medical Imaging Outline of my presentation ✓ T1 recovery. ✓ T2 decay. ✓ Relaxation in different tissues. ✓ Image contrast. ✓ Contrast mechanisms. ✓ T1 contrast. ✓ T2 contrast  T1 Recovery ( spine-lattice energy transfer) The time is takes for 63% of longitudinal magnetization to recover in a tissue Caused by hydrogen nuclei giving up their energy to the surrounding environment or molecular lattice.  T1 Recovery Time The time is takes for 63% of longitudinal magnetization to recover in a tissue T1 Recovery Time Typical T1 recovery times of brain tissue at 1 T.  Relaxation in different tissues T1 recovery and T2 decay are exponential processes with time constants T1 recovery time and T2 decay time, which represent the time it takes for 63% of the total magnetization to recover in the longitudinal plane due to spin– lattice energy transfer (T1 recovery time), or lost in the transverse plane via spin–spin relaxation (T2 decay time). Generally, the two extremes of contrast in MRI are fat and water. Relaxation in different tissues Fat and water Fat And Water Fat molecules contain Water molecules contain atoms of hydrogen two hydrogen atoms arranged with carbon and arranged with one oxygen. They consist of oxygen atom (H2O). Its large molecules called molecules are spaced lipids that are closely apart, and their packed together and molecular tumbling rate whose molecular motion is relatively fast. or tumbling rate is relatively slow.  T1 recovery in fat and water T1 recovery in fat Short T1 recovery in water Long T1 recovery occurs due to hydrogen nuclei giving T1 recovery occurs due to hydrogen nuclei giving up energy up their energy to the surrounding molecular acquired from the RF excitation pulse to the surrounding lattice. Fat has a low inherent energy and easily lattice. Water has a high inherent energy and does not easily absorbs energy into its lattice from hydrogen absorb energy into its lattice from hydrogen nuclei. In water, nuclei. The slow molecular tumbling in fat allows molecular mobility is high, resulting in less efficient T1 recovery because the molecular tumbling rate does not the T1 recovery process to be relatively rapid match the Larmor frequency and does not allow efficient because the molecular tumbling rate matches energy exchange from hydrogen nuclei to the surrounding the Larmor frequency molecular lattice.  T2 decay Time of 63% of the transverse magnetisation to decay due to dephasing. T2* decay T2* decay  T2 decay time Time of 63% of the transverse magnetisation to decay due to dephasing. T2 decay time  T2 decay in fat and water T2 decay in fat Short T2 decay in water Long T2 decay occurs because the magnetic fields of hydrogen nuclei interact T2 decay in water is less efficient than in fat, as the molecules are with each other. This process is efficient in hydrogen in fat, as the spaced apart, and spin–spin interactions are less likely to occur. In molecules are packed closely together, and therefore spin–spin addition, magnetic moments of hydrogen nuclei in water precess interactions are more likely to occur. It also occurs because magnetic much faster than molecular tumbling. As a result, magnetic moments moments of hydrogen nuclei in fat precess at a similar frequency to of hydrogen nuclei dephase slowly, and there is a gradual, rather than molecular tumbling. As a result, magnetic moments dephase quickly, rapid, loss of coherent transverse magnetization. and there is a rapid loss of coherent transverse magnetization. The T2 decay time of water is therefore long The T2 decay time of fat is therefore short  Image contrast The factors that effect image contrast in diagnostic imaging 1. Intrinsic contrast 2.Extrinic contrast Parameters parameters T1 recovery time TR T2 decay time TE Proton density (PD) Flip angle Flow TI Apparent diffusion coefficient (ADC) Turbo factor/ echo train length B value.  Contrast mechanisms High signal Low signal Tissue has a large transvers components Tissue has a small transvers components of in-phase magnetization at time TE of in-phase magnetization at time TE Large signal amplitude is received Small signal amplitude is received by the coil by the coil Bright area on the image Dark area on the image  T1 contrast - As there is more longitudinal magnetisation in fat before the T1 time of fat is shorter than of water RF pulse there is more transverse magnetisation in fat after RF pulse. TR : It must be shorter than the T1 times - Fat therefore has a high signal and is hyperintanse on a T1 of both fat and water , Because of that contrast image neither fat or water has sufficient time to - Water therefore has a low signal and appear relatively hypointanse on a T1 contrast image fully return to B0  T1 contrast - If TR is too long : both the vactors in fat and water return to B0 and fully recover their longitudinal magnetization T1 contrast  T2 weighting or T2 contrast The T2 times of Fat is shorter than that of water The magnitude of trans magnetization of water large Therefor water has a high signal and hyperintense Therefor Fat has a low signal and hypointense on a T2 contrast image  T2 weighting or T2 contrast TE : It must be long enough to give both fat and water time to dephase Image contrast definitions